Rationale

The whale shark species has been split into two major population groups, the Indo-Pacific Ocean and the Atlantic Ocean population, that make up 70% of the species total population. For our analysis, we decided to explore the Atlantic population, specifically in the Gulf of Mexico. The Gulf is a hot spot for feeding and migration and so we hoped to examine the whale shark abundance and its changes here.

First, we analyzed the whale shark observations in the Gulf of Mexico across a 7-year time frame to see if there were any major changes in abundance. We hypothesized that the abundance locations would vary by month depending on migration patterns.

Next, we were curious to see if climate change and rising sea temperatures could have an effect on the abundance of the whale sharks. The whale sharks generally prefer a temperature range of 69-77°F, but could this range change as the sea temperatures rise? We thought that the data may show a shift in location of whale sharks in order to stay in their desired temperature range.

Dataset Information

The Whale Shark Abundance data used in this analysis is from OBIS-SEAMAP. This was one of the only datasets that had multiple records over a long time period which is why we chose to use it. The whale shark data set spans from 1964 to 2010 and has 8,408 records of whale shark sightings. These sightings include chance encounters and individual catalogues. The encounters and photographs were mainly collected by the general public but the database is maintained by professional scientists.

The Sea Surface Temperature data is from the National Oceanic and Atmospheric Administration (NOAA). They have data on the global annual and monthly measure of sea surface temperature anomaly which is how different the temperature is from the average sea surface temperature. The time frame is from 1854 to present day and the data comes from the International Comprehensive Ocean-Atmosphere Dataset (ICODAS). We chose to download monthly data between the latitudes of 20N and 90N because that includes the Gulf of Mexico which is what we are interested in.

Table 1: Information on Datasets
Dataset Name Data Source Data Range Units Variables used
Whale Shark Abundance OBIS-SEAMAP 1964-2010 Number of whale sharks latitude, longitude, date-time
Sea Surface Temperature (Anomaly) NOAA 1854-Present Kelvin year, month, anomaly

Data Wrangling

First the whale shark dataset was filtered by latitude (17 to 30) and longitude (-98 to -79) to only include observations from the Gulf of Mexico. Date and time were separated into different columns and lubridate was used to turn the date into a date class. For the time series and linear regression analyses, we needed the count of whale sharks found each month. This was done by creating a column for just month and year. Then, a new column was created called total_sightings which was grouped by the month-year column. Lastly, the “distinct” function was used to only include the month-year column and sightings once for each month instead of for each day. Since, there is missing data points for certain months, we created a new data set of all the months between August 2002 and December 2009. This data set was then joined to the whale shark data set. All the months that didn’t have data were NA so for the time series the NAs were replaced with zeros.

The sea surface temperature data was wrangling by removing unnecessary columns and the time range was filtered to only include data between August 2002 and December 2009. A month-year column was made similar to the column created for the whale shark data. The sea surface temperature data was combined (using left_join) with the whale shark data using the month-year column and all NAs were omitted.

Exploratory Analysis

Upon beginning the analysis, our main questions were these: has the abundance of whale sharks in the Gulf of Mexico changed over time? If so, did they change in relation to warming sea surface temperatures (SST)? To find out, we first visualized the total sightings of whale sharks in the Gulf of Mexico across our entire study period (2002-2009; Figure XX). This visualization confirmed that a large increase in whale shark abundance appeared to occur after 2008, which gave us the confidence to proceed with our analysis. We then investigated how SST had changed in the Gulf over the course of our study period to confirm that the water temperature had indeed changed (Figure XX). Upon seeing that it had (check this!!), we knew that there was merit to our investigation, and proceeded to look at more nuanced questions within our study.

After seeing that there appeared to be an overall increase in whale shark abundance, we were curious to see if whale shark abundances had changed within the Gulf of Mexico. Was it possible that the warming waters were causing them to potentially migrate to the Gulf in larger numbers? If so, could the changing water have any impact on which region of the Gulf that they favored? We divided the study area into northern and southern halves, along with eastern and western halves, to see if there appeared to be any sort of trade off between the regions over time (Figure XX). The results showed that there was no clear trade off that could be seen. Instead, it appeared that abundance generally increased across all four regions over time. As one last attempt to view any sort of habitat usage trade off, we then compared the abundance of whale sharks over the study period between each latitude parallel of the Gulf of Mexico (Figure XX), along with their abundance between every two longitute parallels (Figure XX). Again, there was no distinct trade off in usage seen; instead, it again appeared that the whale shark abundance simply increased overall in almost every area. This confired that we should not conduct an analysis to see if habitat preferences had changed, and instead simply focus on whether the number of sighted whale sharks had significantly increased over time or not.


Figure 2. Sea Surface Temperature over Time

Figure 2. Sea Surface Temperature over Time

Figure 2 shows that sea surface temperature anomaly is increasing slightly over time.


Figure 3. A four-panel comparison of whale shark sightings in the northern and southern halves of the Gulf of Mexico, followed by the Eastern and Western halves. Each category of sightings had different time periods associated with them, so note the differences in the x axes.


Figure 4. A comparison of whale shark sightings within each latitude parallel of the Gulf of Mexcio.


Figure 5. A comparison of whale shark sightings within every two longitude parallels of the Gulf of Mexico.

Figure 6. This map shows the location of whale shark observations by month

As you can see, the whale sharks seem to be aggregating around the Yucatan Peninsula during the late summer/early fall months and migrating to the northern region of the Gulf during the winter months.

Analysis

Time Series Analysis of Whale Shark Sightings Over Time

Figure 7. The decomposed time series of monthly sightings in the Gulf of Mexico

Figure 7. The decomposed time series of monthly sightings in the Gulf of Mexico

The increasing trend of monthly sightings in the Gulf of Mexico was significant (tau = 0.32, p = 0.002). However, there were not enough sightings per month for the analysis to be valid. Therefore, I aggregated the sightings by year instead and analyzed the time series again.

The results of this analysis did not show that there was an increasing trend using a Mann-Kendall analysis (tau = 0.43, p = 0.17). However, we conducted a linear regression analysis on the data as well, and that test did return positive results (p = 0.04). However, due to the conflicting results of the tests and the p value from the linear regression still being rather large, we felt that a third analysis could improve the strength of our conclusions. Therefore, we isolated the months in which whale sharks were the most frequently seen (June - September; Figure 8), and performed a final Mann-Kendall analysis on those months. The results showed that there was a significant increase in sightings over those “hot spot” months over time (tau = 0.38, p = 0.004).

Fig. 9. Time series of whale shark sightings over the months in which whale sharks were most frequently spotted (June-September)

Fig. 9. Time series of whale shark sightings over the months in which whale sharks were most frequently spotted (June-September)

Map of Monthly Hotspots

Caption: We decided mapping the whale shark abundance by year was not the most effective way to observe the data as there were too many years that had too little observations or only had observations for a single month.

Whale Shark data by Year

Caption: We encourage you to zoom in to the tip of the Yucatan Penninsula to better observe the abundance of whale sharks in this region. As you can see, there seems to be more whale sharks moving south towards the equator during 2009.

Observations in June By Year

Observations in July By Year

Observations in August By Year

Observations In September By Year

Sea Surface Temperature vs. Whale Shark Sightings

Figure 9. Sea Surface Temperature vs. Whale Shark Sightings

Figure 9. Sea Surface Temperature vs. Whale Shark Sightings

Figure 9 shows that there is an increase in whale shark sightings as sea surface temperature anomaly increases. To analyze this increase we used a GLM. The results of the GLM showed that this increase is not significant (p-value = 0.4093, DF=36, R-squared= 0.019).

Summary

Overall, there was a significant increase in whale shark sightings overtime throughout the study area. While this could be an encouraging result that may indicate the whale shark population might be growing, and/or may be using the Gulf of Mexico in higher numbers, there are other confounding variables at play. Given the opportunistic nature of this dataset, it’s possible that there simply may have been more people looking for whale sharks in the later years of the study period. This could be because the whales were in a more favorable environment, or maybe the public was better educated about how to submit sightings to the database managers. In summary, while there was a signficiant increasing trend in sightings over the years, we cannot be sure about how or why that occurred. Conducting systematic surveys looking for whale sharks within the Gulf that have a standardized methodology in the future could help give more confidence in the findings of this analysis, and indicate if there actually are more whales visiting the Gulf of Mexico.

Sea surface temperature did not affect whale shark sightings significantly. This could be because the data we looked at only spanned a few years or because the whale shark data was opportunistic so there may have been more whale sharks than counted.